A common characteristic of interactions between electromagnetic radiation and matter is the production of scatter radiation. Scatter radiation differs from incident radiation in at least one, and usually all of the following; direction of propogation, frequency (photon energy) and phase. In diagnostic imaging using x-ray radiation the scatter radiation has the deleterious effect of contributing to a measurement, e.g., film exposure, without contributing information. This comes about because the objective of diagnostic x-ray imaging is to measure the line integral of composite attenuation through a collection of points in the body along a path parallel to the x-ray beam. When this is done by exposing an area of the body at one time rather than making narrow beam point-by-point measurements, some of the scattered radiation produced reaches the measuring device. However, the point of interaction of the scattered ray with the measuring device bears no relation to the transmitted primary radiation which is being measured at that point. That is, the line integral of attenuation is being measured through primary beam transmission, making use of the fact that the transmitted primary radiation follows the trajectory of the incident beam. Scatter radiation trajectories, however, adhere to this trajectory only by chance. Thus, if the measuring device cannot discriminate between different angles of incidence for the incoming radiation, and cannot discriminate on the basis of photon energy or phase, the resulting measurement becomes corrupted by the contributions of scatter radiation.
Scatter radiation may differ from incident primary radiation in two other respects, photon energy and phase. The fact that scatter photon energies are always less than or equal to the primary photon energy is exploited in nuclear medicine imagery, where monochromatic sources are used and the detectors have an energy discrimination capability. In diagnostic x-ray imaging, however, the source is not monochromatic and energy discrimination is less practical. The typical x-ray tube emits a polychromatic spectrum, even at constant potential with significant filtration. The detectors, whether typical film/phosphor screen combinations, image intensifier tubes, solid-state detectors, or pressurized gases used in some detector arrays, exhibit rather broad spectral energy sensitivities. Even if the source were monochromatic, scatter rejection by pulse-height analysis would be limited with present technologies due to the photon flux rates which must be used in diagnostic imaging. Thus, although scatter radiation in x-ray imaging does exhibit a spectral shift toward lower energies compared to the incident primary radiation, it has not been practical to make use of this fact. The last opportunity for differentiation of scattering from primary radiation, i.e., phase, is precluded by both the lack of a phase-coherent source, e.g., a practical x-ray laser, and the lack of a practical phase-coherent detector. Thus, the primary means used to differentiate scattering from primary radiation has been beam trajectory.
In an area beam detector, which is an x-ray image receptor which can image a relatively large area of anatomy at one time, several schemes have been devised to selectively reduce scatter relative to primary radiation by using the fact that scattered beam trajectories diverge from the primary. Perhaps the simplest, and certainly the most common is the x-ray grid. A grid is simply an array of lead strips, separated by radiolucent material, which is focused on the focal spot of the x-ray tube. The grid is placed perpendicular to the x-ray beam between the patient and the detector. The focusing of the grid acts to preferentially accept primary radiation and reject scatter, due to the fact that the apparent origin of a scattering x-ray photon is not likely to be co-linear with the focal spot of the x-ray tube. While the grid improves the ratio of primary to scatter radiation, the increased x-ray tube power requirements and patient dose are a major disadvantage.
Other schemes to reduce scatter in area beam imaging have been attempted. In the slit-scan and scanning grid method, the x-ray beam is reduced to a slit by appropriate beam collimation, and then scanned across the area of interest. At the same time, and synchronous with the beam scanning, a slit window is scanned between the patient and detector. This method does offer improvements in scatter with minimal impact on patient dose, but x-ray tube requirements and imaging times are significantly increased.
In the techniques employed to reduce scatter, several important considerations arise. First, some compromise between scatter rejection and image acquisition time must be made. The tradeoff will depend on the rate of motion of the anatomy of interest. In early line-scan imaging of the brain, for example, scan times of five minutes were considered acceptable, while in coronary artery imaging, exposures longer than about 8 milliseconds may be unacceptable. Secondly, the finite output power capabilities of x-ray tubes pose practical limitations to narrow-beam and slit scan approaches, which require longer image acquisition times. The extent of scatter reduction which can be achieved in the image acquisition process, therefore, will be dictated by the exposure time constraints of the particular study.
Area detectors offer several significant advantages over line and point scanning systems despite the problems with scatter. These include: simultaneously imaging of large areas of anatomy, short exposure times, high image repetition rates, high utilization of limited x-ray tube power, and relative simplicity. The first four advantages are quite important clinically. The first two--simultaneous imaging of large areas and short exposure times--allow the diagnostician to image moving anatomy without motion blurring and without loss of timing or cause/event relationships. The third advantage--high image repetition rates--allows the assessment of dynamic changes in anatomy. The fourth advantage--power utilization--insures that the images obtained will have sufficiently high photon statistics to allow detection of significant contrast levels and details.
The advantages of the area beam imager lend it to multiple-energy imaging applications, where sets of images are acquired at different beam energies in order to form material-selective composite images. This is well described in the prior art. Scatter interferes with multiple-energy imaging in the ways already described, i.e., in reducing contrast and adding noise. In addition, scatter interferes with the formation of the material-selective image itself, producing the intensity variations which are dependent upon the scatter fraction at each point in the image. To correct the situation, the scatter component of intensity must be removed prior to the combination process. The potential applications of multiple-energy imaging include: angiography, intravenous pyelography, chest radiography, cholangiography and cholecystography, along with various other studies including detection of calcifications and abnormal tissue masses, such as tumor studies. Another potential application of material-selective imaging is bone marrow evaluation. In summary, scattering correction is essential to the process of material-selective image formation in area beam imagers, regardless of whether quantitative analysis of the resulting data is required.
The present invention is related to the prior art lead grid method of scatter correction in that it involves the use of x-ray absorbing test objects in the imaging field. Additional exposures using test objects are required. There are several practical considerations concerning the time in which the test and diagnostic images are acquired. First of all, the time required to gather the test object images must be less than the time in which significant patient motion can occur. Patient motion can be voluntary, respiratory, reflex or organ motion, such as motion of the heart or intestines. There are two approaches to handling organ motion, depending upon whether the motion is cyclic cardiovascular pulsation or random. In the case of cyclic motion of the vascular system, the x-ray exposures can be synchronized using an ECG, so that there is no observable motion between exposures. For the case of random motion, as in the intestines, techniques of motion reduction such as abdominal compression and intravenous administration of glucagon can be used. In all cases, minimizing the total time required for acquiring the test images will be advantageous.
Another consideration is the motion which may occur between the acquisition of the scatter measurement images and subsequent images to be corrected, such as contrast images in angiography. One would want the scatter distribution computed from the test object images to spatially register with the scatter distribution in subsequent images of interest. The scatter distribution is predominately a low-spatial frequency phenomena. That is, the intensity of scatter distribution does not vary significantly over short distances, as would the primary intensity in an area of anatomical detail. Thus, the scatter correction process should be relatively insensitive to small positional shifts in the scatter distribution, and therefore, patient motion. However, to minimize the possibility of such problems, the time between the acquisition of the test images and the actual study images is kept to a minimum. This implies that the scatter calculation and correction process is most effectively done after the total series of images has been acquired when time is not critical.
It is an object of the present invention to provide a method of scatter correction which can be applied after an image or images have been acquired. In this approach a set of test exposures is made to actually measure the level of scatter present at each point in the image. These test images may be acquired before or after the diagnostic image or images are acquired. The effect of the scatter is subsequently removed from diagnostic images by using the information gained from the test images.
It is a further object of this invention to enhance the x-ray imaging process by providing true primary attenuation data in imaging environments where scatter reduction in the acquisition process is limited due to exposure time constraints.